1. Field of the Invention
The present invention relates to an injection mold used to form a resin molded item, and a partial compression method for the injection mold.
2. Description of the Related Art
In a mold for partial compression molding disclosed in Japanese Patent Laid-Open No. 5-228970, depressions are formed by moving ejector pins back away from a mold surface, which forms an inner surface of a molded item, by a compression allowance. The depressions and a cavity are filled with resin in this state, and the resin is then compressed by the ejector pins. Japanese Patent Laid-Open No. 11-198198 discloses that the leading end of a boss is compressed by an ejector pin and that a peripheral portion of the boss is compressed by a sleeve pin. Further, Japanese Patent Laid-Open No. 2002-283422 discloses that an inner portion of a boss is compressed.
A known partial compression molding method will be described below with reference to FIGS. 7, 8, and 9.
In an injection compression mold shown in FIG. 7, a cavity 1 is defined by a cavity mold section 6 and a core mold section 7. The leading end of each ejector pin 17 is held at a position shifted rearward from a mold surface, which forms an inner surface of the cavity 1, so as to form a depression corresponding to a compression boss 4 which is a boss for compression (hereinafter a boss for compression will be referred to as a compression boss). A compression mechanism is provided in a movable clamping plate 10. The compression mechanism includes a plurality of hydraulic chambers 18 and hydraulic cylinders 19 to be hydraulically driven in the hydraulic chambers 18. The hydraulic cylinders 19 are disposed on a rear surface of an ejector plate 9 that is connected to the ejector pins 17. In order to mold an item with the mold having the above-described configuration, filling of molten resin is started in a state in which the ejector pins 17 are shifted rearward from a core surface by the distance corresponding to the compression bosses 4, and the interiors of the cavity 1 and the compression bosses 4 are filled with the resin. After resin filling is completed, the compression mechanism is started so that the ejector plate 9, that is, the leading ends of the ejector pins 17 are pushed toward the cavity 1 (to the right in the figure) by the hydraulic cylinders 19, thus compressing the compression bosses 4. By this compression, the pressure is substantially uniformly transmitted to the entire mold. This can remove a sink mark from a rib. In order to prevent a sink mark from being formed on the rear side of the boss, the leading end of a boss portion 20 can be compressed by a sleeve pin 21 slid by a hydraulic cylinder 19, as shown in FIG. 8, or an inner pin 22 that forms a depression in a boss portion 20 can be compressed by a hydraulic cylinder 19, as shown in FIG. 9.
As described above, the ejector pin retreats by the compression allowance before resin filling, and a compression boss formed during resin filling is compressed, so that a sink mark can be prevented from being formed on the rear side of a boss or a rib.
Unfortunately, in the above-described known methods, since resin flows to the compression boss during resin filling, the resin flow undulates at the leading end on the surface of the cavity, and nonuniform flow easily occurs. When the resin flows to the compression boss, the pressure of resin in the cavity of the compression boss decreases, and the cavity surface of the rear side of the compression boss and a portion downstream of the compression boss become regions having low transferability because of insufficient resin filling. Consequently, an appearance defect, such as nonuniform transfer and uneven glossiness, is sometimes caused. Further, since there is a pressure difference in the cavity after resin filling, when the compression boss is compressed by the ejector pin, the compression pressure varies in accordance with the position of the ejector pin, and it is difficult to apply a sufficient compression pressure to necessary portions. Moreover, since compression needs to be performed with the hydraulic cylinder, the mold structure is complicated and the cost is increased.